Heart-rate is an important parameter which is indicative of the body conditions of a human being. During exercise, sports or athletic activities, it is always desirable to monitor the heart-rate for optimal results as well as for personal safety. The simplest way to measure heart-rate is probably by finger pressing the wrist and then counting the number of heart beats within a given time in order to calculate the heart beat per minute (BPM). However, such a primitive method may not give an accurate result, requires a relatively long pulse-counting period and may not be sufficiently reliable for most practical purposes. To facilitate more accurate and convenient heart-rate measurements, devices with electrocardiographic (ECG) signal processing and measuring means are available.
ECG signals are electrical signals flowing through the heart which are indicative of the electrical activity of the heart and are usually picked up from the skin. Each typical and complete ECG signal or electrocardiogram includes a complete waveform with the more salient labels P, Q, R, S and T indicating the more distinctive and significant features of the waveform. It is generally recognized that the P wave arises from the depolarisation of the atrium, the QRS complex arises from depolarisation of the ventricles, and the T-wave arises from re-polarization of the ventricle muscle. The magnitude of the tall, spiky R-wave of the PRS complex is approximately 1 mV. When the heart beats, a train of repetitive ECG signals with the characteristic P-QRS-T waveform can be detected. The instantaneous heart-rate can be determined from the period of the train of ECG signals, for example, by measuring the time difference between immediately adjacent spiky R-peaks of the train of the ECG signals.
In order that the heart-rate can be determined by automated ECG analysis for enhanced accuracy, sensitivity, convenience as well as within a shorter time, devices with automated ECG analysis capability are required. ECG signals are usually detected by applying electrodes to the skin, usually also in the presence of noise. Typical examples of noise sources which are commonly known to corrupt ECG signals include, for example, power line interference, electrode contact noise motion artefacts, muscle contraction (electrode myographic, EMG), based line drift and ECG amplitude modulation with respiration, instrumentation noise generated by electronic devices, electrosurgical noise and other, less significant noise sources. The nature and significance of such noise sources have been extensively studied and discussed in many publications, including, for example, in the journal article entitled “A Comparison of the Noise Sensitivity of Nine QRS Detection Algorithms” by G. M. Friesen published in 1990 IEEE Trans-Actions on Biomedical Engineering Vol. 37, No. 1., which is incorporated herein by reference. As the ECG signal data received from the skin are usually contaminated with noise, heart-rate measurement devices equipped with ECG signal analysing means always include noise filtering means or algorithms in addition to ECG signal processing and analysing means or algorithms. Digital signal processing techniques are frequently used to perform noise filtering as well as ECG signal processing and analysis because of the many different types of noise as well as the rather complicated ECG signal waveform. However, conventional noise filtering and ECG signal processing techniques are very complicated and require substantial computational overhead which usually means a rather long computational time as well as a high energy consumption.
As people are becoming more health conscious, the demand for portable heart-rate monitoring or measuring devices or apparatus have significantly increased. A wrist-worn type heart-rate monitor which is usually incorporated as part of a wrist-watch is a good example of such portable heart-rate monitoring or measuring devices. A typical wrist-worn heart-rate monitoring watch usually includes a wrist-strap, a watch casing with a conductive back cover, an ECG sensing electrode mounted on the watch casing and a digital display panel for displaying the time-of-the-day and the heart-rate in BPM. As a wrist-worn heart-rate monitoring watch is usually powered by a single button cell to attain light weight and a compact design, it is highly desirable if the underlying noise filtering and ECG signal processing algorithms or means do not require excessive power consumption to extend battery life. Examples of wrist-worn heart-rate monitoring watches are known, for example, in U.S. Pat. No. 5,289,824 and U.S. Pat. No. 5,738,104. In the wrist-worn heart-rate monitoring watch disclosed in U.S. Pat. No. 5,289,824, the incoming ECG signal data have to pass through five filtering stages before subjecting to a QRS complex detection and validation process in order to determine the heart-rate.
U.S. Pat. No. 5,738,104 also discloses a wrist-worn heart-rate monitoring watch including two stages of digital filtering, namely, a first stage of a low pass filter and a second stage of band-pass filter. The digitally filtered ECG signal data are then subject to an enhancement signal processing block which includes a differentiation step followed by a squaring or absolute value operation and are then subject to the calculation of the moving average. A template-matching or cross-correlation process on the digitally filtered incoming signal data is then performed to compare or cross check against the results of the enhancement signal processing. The resulting digital data are then analysed to determine the users' heart-rate. However, the algorithms utilized in most known wrist-worn type heart-rate monitoring watches are often not sufficiently power- and time-efficient to satisfy's increasingly stringent consumer demands. Hence, it is highly desirable if there can be provided improved ECG signal processing means or algorithms for heart-rate determination with a reasonable accuracy and a reasonable power- and time-overhead. Thus, it will be highly desirable if there can be provided simplified schemes or methods for heart-rate measurements suitable for portable, low-power, applications.